1. Field of the Invention
The present invention relates to an Ethernet passive optical access network (EPON), and more particularly, to a device and method for allocating a bandwidth to a plurality of optical network units (ONU) for transmitting an uplink frame in the EPON where a plurality of ONUs is connected to one optical line terminal (OLT).
2. Description of the Prior Art
Generally, an access network is a communication channel between a service provider, such as a central office CO, a Head-End or a point-of-present (POP), and subscribers that receive services from the service provider.
Recently, an xDSL network and a Hybrid Fiber Coaxial (HFC) network are mainly used as the access network, wherein the xDSL may be an asymmetric digital subscriber line (ADSL) or a very high data rate digital subscriber line (VDSL) that mainly delivers low-speed data and voice data, and the HFC network may be a cable broadcasting network. Due to the development of high speed Internet, high speed local area network (LAN) and Home network and the introduction of various applications for voice, data and multimedia stream, the bandwidth required for the access network has been rapidly increasing, and the xDSL network using a typical telephone line and the HFC network using a coaxial cable have failed to accommodate the requirement of bandwidth of the access network.
As described above, various services, such as video on demand (VOD), cable television (CATV) and high definition television (HDTV), have been explosively increasing, and an access network using an optical line has been noted for an environment of a broadband convergence network (BcN) where high-speed Internet traffic and communication and broadcasting data are merged. Especially, a passive optical network (PON) has been recognized as the most suitable alternative for a Fiber To The x (FTTx) in economical, technological and evolutional view of a communication network.
Although the technology of the PON was developed for an asynchronous transfer mode (ATM) network in its early stages, the ATM based PON (APON) was not widely used because of several shortcomings, such as high complexity, high cost, increment of Ethernet based access traffic and difficulty of accommodating video service. Recently, the standard of an Ethernet PON (EPON) was completely defined based on a task force of IEEE 802.2ah Ethernet in the first mile (EFM). Since the EPON technology transmits an Ethernet frame through a passive optical line between the central office (CO) and subscribers without changing a protocol, the EPON technology has the advantage of price competitiveness compared to the telephone or the coaxial cable based network. Due to such an advantage, many researches have been recently progressing for developing the EPON technology, and various manufacturers have been producing chips conforming to the EPON standard. Especially, the optical access network employing the EPON has been recently widely spread.
FIG. 1 is a block diagram illustrating a structure and an operation for transmitting uplink data and downlink data in an Ethernet passive optical network (EPON) according to the related art.
Referring to FIG. 1, the Ethernet passive optical network (EPON) includes an optical line terminal 100, a splitter 105 and optical network units (ONU) 110, 120 and 130. The EPON has a point-to-multipoint structure where a plurality of optical network units 110, 120 and 130 share the optical line terminal 100 through a single optical line. That is, downlink transmission transmits data from the OLT 100 to the ONUs 110, 120 and 130 based on a broadcasting scheme. On the contrary, uplink transmission transmits data from a plurality of the ONUs 110, 120 and 130 to the OLT 100 based on a multipoint-to-point scheme.
The optical line terminal 100 is located at the center office (CO). The optical network units (ONU) 110, 120 and 130 concentrate subscriber frames generated from subscriber terminals 140 to 145. The subscriber terminals 140 to 145 may be connected to a home gateway (HG) placed inside a house, and the home gateway may be connected to a set top box (STB), a personal computer (PC), the internet telephone (VoIP) or etc. The splitter 105 forms an optical distributing network between the OLT 100 and a plurality of the ONUs 110, 120 and 130 by connecting the single OLT 100 to a plurality of the ONUs 110, 120 and 130 in a one-to-N manner. In the EPON, the downlink frame 160 is processed similarly to a conventional Ethernet frame in that the downlink frame 160 is transmitted using the broadcasting scheme. However, a plurality of ONUs 110, 120 and 130 transmit the uplink frame 150 to the single OLT 110. For example, one ONU 110 shares network resources with other ONUs 120 and 130 in the EPON in order to transmit the uplink frame L1, and the OLT 100 controls the ONUs 110, 120 and 130's rights to access the shared network resource. Therefore, in a tree-structured EPON, a fair and effective bandwidth allocation method and a multi point control protocol which is a MAC protocol are used in order to prevent collision which may occur in the uplink data transmission and allocate a bandwidth. In the uplink data transmission, the simplest method of allocating the bandwidth is a static bandwidth allocation (SBA) method that equally allocates same bandwidths to all of registered optical network units 110, 120 and 130. However, the static bandwidth allocation (SBA) method has disadvantages in that the SBA method has difficulty to support the Quality of Service (QoS) and the SBA may not allocate bandwidth differently according to the optical network units 110, 120 and 130 although there is enough margin in the entire bandwidth. In order to overcome the disadvantages of the SBA method, dynamic bandwidth allocation (DBA) methods were introduced, and an interleaved polling with adaptive cycle time (IPACT) method is the representative method for the DBA methods. For example, in IPACT method, the OLT 100 transmits a GATE message to a next ONU 120 using the downlink transmission before the ONU 110 having the current right of transmission ends the last transmission. That is, the IPACT method can perform the dynamic bandwidth allocation by informing the OLT 100 of buffer information of the ONUs 110, 120 and 130 when each of the optical network units 110, 120 and 130 transmits the uplink data according to the gate message transmitted from the OLT 100. Such an IPACT method has an advantage in that the available of link is improved using statistical multiplexing. However, the IPACT method has difficulties to provide a service that is very sensitive to jitter or delay because a polling cycle varies. Accordingly, researches for developing an allocation scheme using a regular polling have been attempted to support QoS. However, the allocation scheme using a regular polling may waste the bandwidth because the bandwidth is allocated in advance.